The ferrocyanide/ferricyanide redox couple, Fe(CN)63−/4−, is well understood and is frequently used in energy storage applications, but the low solubilities of available salts has limited its use, owing to the low associated energy densities. For example, the solubilities of Na4Fe(CN)6.10H2O, K4Fe(CN)6.3H2O, and Ca2Fe(CN)6.11H2O in water at ambient temperatures are listed in Ullmann's Encyclopedia of Industrial Chemistry as 33.7 g, 33.7 g, and 148.4 g in 100 g of water, respectively (other sources list similar or lower values for these solubilities). These correspond to concentrations of about 0.7 M, 0.8 M, and 3 M, respectively. Given these limits, energy storage systems use the Fe(CN)63−/4− couple at concentrations lower than these at ambient temperature at all pH ranges (and are typically not greater than 0.52 M). While the use of alkaline earth metal salts may provide higher concentrations at neutral pH, their use in alkaline systems is disfavored by the precipitation of metal hydroxides—e.g., Ca(OH)2.
Prior efforts to use this ferrocyanide couple in energy storage systems generally seek to overcome the inherent solubility limits of Na4Fe(CN)6 or K4Fe(CN)6 systems by engineering means and/or by operating systems at elevated temperatures. For example, one group explored the use of elaborate flow-through crystallizers in the electrolyte stream to increase the energy density of the solution from the 0.5-0.6 M [Fe(CN)6] dissolved in the liquid phase by separating out insoluble crystallites. See Hollandsworth, R. P., et al., “Zinc/Ferrocyanide Battery Development Phase IV” Lockheed Missiles and Space Company, Inc., contractor report, Sandia Contract DE-AC04-76DP00789, 1985.